1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device by photolithographic method using extreme ultraviolet (EUV) light, especially light having wavelength in a soft X-ray region, a reflection type photomask and a reflection type photomask blank used in this method.
2. Description of the Related Art
In the recent trend of high degree of integration in semiconductor elements, there is an increasing demand for miniaturization of necessary pattern transfer on Si substrate by photolithography.
In the conventional method of photolithography using a lamp light source (wavelength of 365 nm) or an excimer laser light source (wavelength of 248 nm or 193 nm), shortening of wavelength of light source is nearing the exposure limit. There is hence an urgent need to establish a new method of photolithography capable of processing as finely as 100 nm or less.
In the development of photolithography, it has been promoted to use F2 laser light (wavelength of 157 nm) which is excimer laser light in a shorter wavelength region. However, since the half size of exposure wavelength is usually the substantial limit of development, the processing limit is about 70 nm.
As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-237174, a new method has been developed, that is, EUV lithography using a light source of EUV light (wavelength of 13 nm) having a wavelength more than one digit shorter than in F2 laser light, specifically 10 to 15 nm.
Refractive index of substances in a wavelength region of EUV light is only slightly smaller than 1. In this method of EUV lithography, since a refractive optical system cannot be used unlike in the conventional exposure light source, a reflective optical system is used for exposure. In the wavelength region of EUV light, almost all substances have a high light absorbing property. Therefore, as a photomask for pattern transfer, a reflection type photomask is used instead of the existing transmission type photomask. Thus, in the method of EUV lithography, the optical system and the photomask used in exposure are evidently different from those used in the conventional exposure technology.
This reflection type photomask for the EUV lithography is composed of a flat Si substrate or a composite quartz substrate, a mirror. (reflection mirror) having a high reflectivity in an EUV wavelength region formed thereon, and a light absorption layer of heavy metal having a particularly high absorbing property of EUV light formed further thereon, being patterned according to a desired exposure pattern.
The mirror (reflection mirror) reflecting the EUV light is a multilayer reflection film composed of materials largely different in refractive index. The refraction type photomask transfers the exposure pattern by contrast of EUV exposure reflectivity between the absorption region having the multilayer reflection film surface covered with the light absorption layer, and the reflection region having the multilayer reflection film surface exposed without the light absorption layer.
Usually, the pattern formed on the light absorption layer is inspected by emitting deep ultraviolet (DUV) light having wavelength of about 190 to 260 nm to the mask surface, detecting the reflected light, and measuring the contrast of reflectivity. Specifically, the surface of a buffer layer which is provided immediately beneath the light absorption layer as a protective layer of the multilayer reflection film is used as a reflection region. Then, before processing the pattern of the light absorption layer, based on the contrast of reflectivity with the absorption region of the patterned light absorption layer surface, it is inspected whether the light absorption layer is patterned and formed as desired. In this first step of inspection, it is intended to detect whether the light absorption layer to be removed by etching is left over on the buffer layer (black defect), and whether the light absorption layer to be left over on the buffer layer without being etched is partly removed by etching (white defect).
After correcting the defect detected in the first step of inspection, the buffer layer is removed, and the surface of the multilayer reflection film beneath the buffer layer is exposed, after which the pattern formed in the light absorption layer is finally inspected in the second step. The final inspection is performed by observing the contrast of reflectivity between the absorption region composed of the light absorption layer surface and the reflection region of the multilayer reflection film surface. It may be inspected without removing the buffer layer, but the buffer layer is often removed because the existence of a coated film of the buffer layer on the multilayer reflection film surface tends to lower the reflectivity of the multilayer reflection film.
The inspection of the light absorption layer pattern by DUV inspection light in the first and second steps is intended to observe the contrast of DUV light reflectivity between the buffer layer surface from which the light absorption layer is removed and the light absorption layer surface on which the light absorption layer remains without being removed, and between the multilayer reflection film surface from which the buffer layer is removed and the light absorption layer surface. Therefore, to enhance the precision of inspection, it is desired that the difference in reflectivity is greater in the DUV inspection wavelength region, between the buffer layer surface and the light absorption layer surface in the first step, and between the multilayer reflection film surface and the light absorption layer surface in the second step.